Interest in recovery CO2 from various CO2 containing gas mixture is propelled by multiple factors: the merchant CO2 market, enhanced oil recovery (EOR) and greenhouse gas emissions reduction. However, majority of CO2 sources are from low pressure gas mixtures having relatively low concentration of CO2. Such sources, for example, include the flue gas from a fossil fuel-fired power plant, an industrial furnace, a cement kiln, or an oxy or air combustion facility, or the exhaust gas of an engine or lime kiln. Typically, the flue gas is obtained at near ambient pressure (<3 bara). The concentration of CO2 in the flue gas ranges from approximately 5 to 25%, with a balance of mostly nitrogen. The flue gas flow rate is numerous.
Conventionally, most commercial CO2 recovery plants use processes based on chemical absorption with a monoethanolamine (MEA) solvent. MEA was developed over 60 years ago for removing CO2 and H2S from natural gas streams. However, the process suffers large equipment costs and high regeneration energy requirements. Recently, a CO2 CPU (compression and purification unit) process was proposed to capture the CO2 from SMR (steam methane reforming) plant H2 PSA (pressure swing adsorption) off gas. The benefit of the process is that the waste gas from the CPU plant, which normally contains significant amount of H2 at high pressure, can be recycled back to the PSA for additional H2 production credit. But the CPU process requires high compressing and cold temperature operation, especially, when the CO2 concentration in the feed to CPU process is low, such as in the case of flue gas. The application of the CPU process in CO2 capture is economically adjustable only when the feedstock contains relative higher amount CO2 concentration, for example 50%.
Recovery CO2 from flue gas sources by using adsorption technologies was proposed earlier. Paul Webley's group used vacuum swing adsorption (VSA) process to recover CO2 from power plant exhaust gases by applying conventional 13× adsorbent. However, the use of a vacuum pump limits the process to be operated at a very fast cycle time and therefore the productivity remains relatively low. Energy consumption is high due to the necessity of vacuum. On the other hand, thermal swing adsorption (TSA) processes were also proposed by many groups to remove CO2 from air or flue gases. For example, there are studies in operating TSA at relative short cycle by designing adsorbers as a heat exchanger type (including heat exchange tubes inside adsorbent bed or coating adsorbent onto surface of heat exchange tubes) to increase heat transfer in and out of adsorbents. But the amount of adsorbent loading per unit volume is low leading to large and costly adsorber even at reduced cycle time. Therefore, there is an urgent need for developing a technology which can capture CO2 from low pressure sources having CO2 concentration in a range of 5 to 30% more economically.
The proposed invention is to design a rapid cycle thermal swing adsorption process to capture CO2 from low pressure and low concentration CO2 sources utilizing the novel structured adsorbent bed configuration with electric energy to regenerate the adsorbent bed. This new compact design allows operating the thermal swing adsorption process at a much faster cycle speed than the conventional pellet-loaded adsorbent bed, therefore to significantly increase CO2 production yield. The design of new rapid cycle thermal swing adsorption (RTSA) process has advantages of the same or even higher overall adsorbent loading per volume compared with beaded materials in enough mass transfer rate, but with much lower pressure drop and in much fast heat transfer rates by reducing characteristic lengths of the transport distances at all scales of the adsorption and desorption process steps. As a result, the proposed technology ideally suits applications that involve large gas flow and are sensitive to pressure drop. Success of this technology for CO2 capture application will open up many other opportunities, such as driers for conditioning, natural gas upgrade, etc.